1. Field of the Invention
The present invention relates to a microcontroller (MCU, Micro Controller Unit), and more particularly, to a microcontroller used in the receiving part of a system (a remote control system, etc.) processing a feeble electric wave.
2. Description of the Related Art
Recently, the development of a system having a remote operation function in wireless is in progress. For example, a system, which includes a function of starting the engine and a function of finding the location of an automobile as well as an unlocking/locking function through remote operation, has also been developed in a keyless entry system of an automobile. In such wireless remote operation, as the distance between a transmitter unit and a receiver unit becomes more distant, an electric wave becomes feeble on the part of the receiver unit. Thus, although there is a little noise (EMI, Electro Magnetic Interference), an original electric wave is buried in the noise. For this reason, the sensitivity of the receiver unit is lowered. Consequently, if the distance between the transmitter unit and the receiver unit is not reduced, there is a problem in that the remote operation cannot be normally performed. Accordingly, lowering the noise of the receiver unit has been an important issue in the system whish has the remote operation function in wireless.
For example, the receiver unit in the keyless entry system of the automobile includes a RF receiver LSI that demodulates a RF signal and a microcontroller that processes the demodulation signal generated by the RF receiver LSI. The RF receiver LSI and the microcontroller are mounted on the same system board. In this receiver unit, in order to prevent the receiving sensitivity of the RF receiver LSI from lowering due to noises generated from the microcontroller, the following noise measures are taken.
In first noise measures, when the operation frequency (the frequency of an operating clock) of the microcontroller is a fundamental frequency f1 (the frequency of a fundamental wave), the noise is generated in harmonics (2×f1, 3×f1, 4×f1, etc.) whose frequency is an integral multiplication of the fundamental frequency f1. Therefore, The operation frequency of the microcontroller or a receiving frequency band are set so that these harmonics are not included in the receiving frequency band of the RF receiver LSI. For example, when the receiving frequency band of the RF receiver LSI is 314 to 316 MHz, if the operation frequency of the microcontroller is set to 15.75 MHz, 20-order harmonic (20×15.75 MHz=315 MHz) is included in the receiving frequency band. Meanwhile, when the operation frequency of the microcontroller is set to 15.85 MHz, any one of the harmonics (19-order harmonic: 19×15.85 MHz=301.15 MHz, 20-order harmonic: 20×15.85 MHz=317 MHz) can be not included in the receiving frequency band.
In second noise measures, when the frequency of an external output signal (a signal output from an external terminal) of the microcontroller is a fundamental frequency f2 in the same manner as the operation frequency of the microcontroller, the microcontroller operates so that harmonics whose frequency is an integral multiplication of the fundamental frequency f2 are not included in the receiving frequency band of the RF receiver LSI. In third noise measures, physical noise measures are given to the system board on which the RF receiver LSI and the microcontroller are mounted. Specifically, lowering noise can be accomplished by forming a GND layer through multi-layering of the system board, by blocking noise from the microcontroller toward the RF receiver LSI through installation of a metal cover surrounding the RF receiver LSI, or by using a lot of noise-prevention components (a condenser, an inductor, etc.).
Further, Japanese Unexamined Patent Application Publication No. Hei8-289372 discloses a technology in which a remote control signal from an electronic key can be obtained reliably without being influenced by noise in a keyless entry system of an automobile.
In the above-mentioned the third noise measures, however, since physical noise measures have to be taken against the system board, the cost of the receiver unit (i.e., the cost of the system) increases. Although the first to third noise measures are taken, an noise reducing effect is not sufficient. Accordingly, it is necessary to develop the system that the receiving sensitivity of the RF receiver LSI, i.e., the distance between the transmitter unit and the receiver unit, is sacrificed, which can be remotely controlled, by some degree.